For micro/nanofabrication of Gallium nitride (GaN), we developed wet-chemical-assisted fs laser ablation in which the femtosecond (fs) laser beam focused using an objective lens was directed on the surface of a GaN substrate immersed in 35% hydrochloric (HCl) acid solution. Nanocrators with a diameter as small as 320 nm (FWHM) were successfully formed on surface of a single-crystal GaN substrate by a single pulse of the second harmonic fs-laser beam (λ = 387 nm, 150 fs) focused using an objective lens with NA of 0.5. Nano scale ablation is responsible for two-photon absorption of the fs laser. The ablated craters exhibit higher quality and better uniformity with little debris formation compared with those produced by fs-laser ablation in air followed by etching in HCl (two-step processing method). The high quality ablation is presumably due to photochemical or photothermal reaction of HCl solution with ablated materials, resulting in complete removal of debris and in sharp edge and smooth surface of craters. We have demonstrated formation of 140-μm-long straight and hollow channels embedded in GaN by scanning the laser beam inside the substrate. 3D micro and nano fabrication technique of GaN has great potential for manufacture of highly-functional micro devices. We have also tried to fabricate 2D periodic nanostructures on GaN surface by scanning the sample in x-y plane. Nanocrators with uniform size periodically arranged on GaN surface can act as a two-dimensional (2D) photonic crystal which is expected to enhance a light extraction efficiency of blue LED.